1. Field of Invention
A pump station flowmeter is disclosed which includes a volumetric flowmeter for determining an accurate measurement of the flow of liquid through a liquid system like a sewage system, a pump status converter for eliminating the usual heavy modification of the control panel required by the installation of volumetric flowmeters, a flow rectifier to calculate flow with maximum accuracy according to each specific installation, an abnormal pump operation processor to differentiate pump flow error calculation from real abnormal pump operation, and a pump flow variation alarm gate for generating alarms when abnormal pump flow is calculated based only on pump-on and pump-off inputs.
2. Description of Prior Art
Most Prior Art is related to volumetric flowmeters specially designed for pump stations. They require level inputs to work. None calculates flow without being connected to level sensors, none individually optimizes its calculus for each specific installation, and none is able to generate pump flow variation alarms without any inputs other than pump-on and pump-off.
Pump station flowmeters, or the like, are well known in the patented prior art, such as the U.S. Pat. Nos.: Martig, Jr. 4,127,030, Jorritsma 4,455,870, 4,669,308 and 4,821,580; Olsson 4,467,657, Free et al. 4,897,797, Hon 4,856,343, Adney 4,962,666, and Marsh et al. 5,313,842 and 5,385,056
(a) All the instruments using these patents must be physically connected to level sensors and/or pressure devices in order to be used. This makes the installation of these instruments laborious and expensive.
Pump stations are composed of a wet well which accepts liquid inflow and temporarily stores such inflow, and a pump, or combination of pumps, which discharge the accumulated liquid from the wet well. The rate of change of level in the wet well with respect to time (dl/dt) is a function of the shape and size of the wet well, the flow entering the wet well is the inflow, and the flow leaving the wet well is the outflow.
The shape and size of the wet well are usually known, and therefore if dl/dt is measured, it can be converted to the rate of change of volume with respect to time (dV/dt). The rate of change of volume depends on the inflow and outflow.
While the pump (or pumps) is off, the inflow can be measured by timing how long it takes for the liquid to fill a known volume. This is done by using existing upper and lower limit switches which are already present to turn the pumps on and off. This method provides an average inflow (Inflow) over the time that it takes to fill the known wet well volume.
If the Inflow is also known during the pump on time, the total volume passed through the wet well in one wet well pump cycle can be calculated by the equation: Outflow=Vp/tp+Inflow.Outflow is the average outflow of the pump in operation for that cycle, Vp is the volume of the wet well between the pump on and pump-off switches, and tp is the length of time the pumps were on. It is important to note that the inflow is a function of time and is not a constant. If an Inflow for the pump-on time is known, then a numerical version of equation Outflow=Vp/tp+Inflow would be used: EQU Volume/cycle=Vp+Inflow.times.tp.
(b) Unfortunately, timing the wet well as it fills will give an Inflow which is not an accurate estimate of inflow during the pump on time if the inflow significantly increases or decreases between the pump off time and pump on time. One way to reduce this error is to add another level switch at an intermediate level to define another, smaller volume. The fill time of this intermediate volume can be used to measure an Inflow over a shorter period of time which is closer to the onset of the pump-on time, and, hence, is a better estimate of the Inflow during the pump on time.
Variations on this idea include measuring the Inflow before and after the pump on time and calculating their average which is Inflow, or adding more intermediate level switches to measure several Inflows and then performing a best fit of the Inflow versus time for interpolation and averaging. Although these techniques improve the system's performance, a sudden flow change will still lead to large errors, and the installation process becomes impractical.
(c) Inflow changing at a high frequency can cause large errors in the flow calculus. High frequency inflows are flows which change a significant amount over a short period of time, making it difficult for a system which samples the flow periodically to obtain an accurate estimate of the inflow during the pump-on time. High frequency flows are likely to occur at pump stations downstream of another pump station or at industrial pump stations. Small domestic pump stations may have high frequency phenomena as well.
A method disclosed in the Jorritsma U.S. Pat. No. 4,455,870 samples the inflow once per pump cycle, and a second method samples the inflow twice per pump cycle, and therefore, it was thought to be twice as accurate as the first method. Adding more intermediate switches allows a system to measure the volume through the wet well accurately at even higher inflow frequencies. However, it is not practical to measure high frequency inflows in this manner because too many switches are required, and the errors related to the sensors themselves add-up.
(d) One important phenomenon of periodic flow entering a pump station can be termed "lock-on". Lock-on occurs when the pump-on time and the inflow synchronize and remain that way which means the liquid is going in at about the same speed it is going out. Lock-on maximizes the errors in flow measurement systems which use fill times to estimate the inflow during the pump on time. The occurrence of lock on is affected by the size of the wet well, the inflow frequency, the inflow magnitude, and the pump characteristics. It occurs very easily over a relatively wide range of frequencies. Such frequency conditions often exist downstream of another pump station or at relatively small pump stations.
Once a pump station is locked on, it will remain so until the inflow frequency changes enough to disturb it. The tendency of the pump-on time and the maximum inflow to remain locked in phase can be explained as follows. At low inflows, the pump is less likely to come on because the level is less likely to reach the top level switch. Conversely, the pump is more likely to come on when the inflow is high. This tendency forces the pump to turn on during the increasing part of the inflow cycle. The pump-on time lengthens because of the increasing inflow. Ultimately, the pump-on time straddles and then passes the inflow peak. Once the pump on time occurs during the period of decreasing inflow, the pump flow is large enough to empty the wet well before the inflow reaches its minimum. At this point, the two cycles are locked in phase and the pump-on time will not advance across the inflow minimum. Under these conditions, inflow estimates based on prior fill time data will be highly inaccurate.
(e) These problems are partially overcome by using a different approach. If the outflow of the pump (or pumps) and the time of operation of the pumps are known, the volume passing through the wet well in one pump cycle can be calculated by: Volume per cycle =pump outflow.times.time of operation. The filling time of the well being known, the Inflow of a cycle can be calculated by: Inflow=volume per cycle/(time of operation+filling time).
In most cases, wet well pumps discharge into an open channel pipe which carries the liquid downstream by gravity: the pumps simply lift the water a constant distance from the pump outlet to the elevation of the open channel pipe. The pump outlet is under a constant pressure due to the column of water between the pump outlet and the beginning of the open channel flow line where the liquid discharges to a gravity feed line. The pressure on the inlet side of the pump is directly related to the level of the liquid in the wet well. The liquid level changes from the pump stop level to the start level to the stop level at each cycle. Each level being reached at each cycle, we can conclude that a constant average pressure generates a constant average pump outflow. If the inflow is accurately calculated, the outflow calculated will be fairly constant. If the calculated outflow is not constant, we can assume two possibilities: the inflow was not properly calculated or the pump outflow had really changed.
Marsh U.S. Pat. No. 5,385,056 assume only the first possibility by comparing the last calculated Outflow of a pump to the average of all the Outflow for that pump which is Outflow. If the Outflow is within a specified range of Outflow, then Outflow is updated with Outflow. If the Outflow calculated is outside the specified range, then Outflow is used instead of the last Outflow. The possibility that an outflow can change drastically, like when a pump is damaged or blocked was not considered. It is more accurate to say that the exact Outflow is somewhere between the last calculated Outflow and Outflow.
Furthermore, they assume that by adding intermediate levels, they would gain accuracy. Level sensors operating in pump stations wetwell are rarely highly accurate due to turbulent liquid surface, grease build-up, solids, etc. Adding levels means less distance between levels. Reducing the distance between levels by two is like doubling the resulting sensor related error. For example, let's say a station using float switches has an accuracy of 1/2 inch each. Two sensors are necessary to calculate a volume. If 20 inches separates the 2 switches, then the error is 5% (1/2.times.2/20"). If an intermediate switch is installed 8 inches from the top switch, the error becomes 12.5% (1/2.times.2/8").
(f) Each pump station being different with its own filling and emptying characteristics, a specific range, common to all stations can not generate the most accurate values for all stations. The station's characteristics change over time, ruling out the possibility of using a specific range even within a station as specified in Marsh U.S. Pat. No. 5,385,056.
(g) The stability of the outflow calculated for each pump, which is calculated using the inflow, is a proof of the accuracy of the inflow. Two reasons can create rapid outflow changes that could let us believe the inflow causes the errors in the calculation. One is natural, meaning inhuman factors cause it, and one is human, meaning the level sensors or the pumps are manually activated because the pump station is in a period of maintenance. Usually in a period of maintenance, the wet well is cleaned using high pressure water. This makes the level detector, specially if floats are used, send false signals to the control panel which starts and stops the pumps at any level at any time. This induces errors in the volumetric flowmeter which understands that the start and stop levels were reached in a matter of seconds generating gigantic inflow and outflow. The maintenance people might turn the pump off or on to determine if they are working properly, which gives the impression that the starting or stopping levels were reached. None of the above patents have any way to detect that a pump station is in a maintenance period.
(h) All the above volumetric methods calculate flow using functions that assume a constant Inflow or average outflow. The real inflow entering the pump station is always changing. This fact invalidates the uses of a constant inflow calculation as an acceptable representation of reality.
(i) Most of the instruments using the above volumetric methods generate alarms based on low or high pump outflow which indicate a pump problem. To do this, the minimum information supplied by the user to the instrument is the geometry of the well, and the instrument must be connected to the level sensors. They can not generate abnormal pump flow alarms without them. It is not practical for a pump manufacturer to integrate in its pumps an outflow alarm system without knowing if the end user will be able to provide the wet well geometry and the level sensors.
(j) No Prior Art shows how to calculate inflow and outflow when a pump is continuously running and when more than one pump is running.
(k) This device can be used in any installation that has a mechanism that changes its state at set levels. This apparatus can facilitate the installation of instruments that need to know the level to operate. Volumetric flowmeters are good examples of these instruments. This apparatus reduces installation time of such instrument from hours to minutes by reducing or eliminating the necessary modification of the control panel of the pump station.
The present invention was developed to avoid the above and other drawbacks of the prior systems.